Logging While Drilling (LWD) in oil and gas exploration refers to the technologies to constantly measure geophysical information around borehole in the process of drilling, and to real-time transmit the logging data back to the surface. One of the key technologies of LWD is the real-time signal transmission. Currently, mud-pulse telemetry system is widely applied, in which the signals are sent to the surface with the help of pulses transmitted through the drilling mud. The mud-pulse telemetry system is used in a wide range of drilling wells, but with a fairly low data transmission rate, usually less than 8 bit per second. Electromagnetic (EM) telemetry technology was developed to improve the transmission rate, which sends signals from the down hole to the surface through EM waves. A short insulation section is installed between the non-magnetic drill collar and the upper drill pipe, and a low frequency alternative current is applied on both ends of the insulation section to generate the EM field. This EM filed contains the logging information, and will be achieved by measuring the voltage between the surface pipe and an electrode, which is set on a remote ground surface. FIG. 1 shows a typical EM telemetry system of a prior art. An insulating ring 2 is set on the drill pipe 5 (on the top of the drill bit 4) to divide the drill pipe into two mutually insulated segments. Power source 3 loads a low-frequency current to both ends of the insulating ring. As the arrows in the figure indicate, the formation (refers to the formation from the vicinity of both ends of the insulating ring 2 to infinity) and the power source form a current loop. In the FIG. 1, the arrowed lines indicate the current distribution in the formation at a certain moment. Wherein, thick lines indicate strong current intensity, thin lines imply weak current intensity, and dotted lines stands for very small current intensity. The currents in the different part of formation change both value and direction with respect to time, but the ratios among the current intensity are substantially constant. As shown in FIG. 1, there is relatively larger current intensity in the sub-loop near the insulating ring 2, and much weaker current intensity in the sub-loop far from the insulating ring 2. And when the well is deep (such as a well 2000 meters long), the current in the sub-loop through the vicinity of surface will be very weak. The LWD data is loaded on the power source, and will be transmitted to the surface through the electromagnetic field generated by the currents. A signal receiver 10 is connected between the terminal A (surface part of casing 1 of the drilling well) and a far ground terminal B. Thus, the downhole data can be acquired by measuring the voltage between terminal A and B using the signal receiver 10. However, because the EM signal strength near the ground surface is very small, the signal receiver 10 is required of an extremely high sensitivity, and the data transmission rate is also quite limited.
As one of wild applied technologies of LWD, Case ranging is the process to locate the downhole casings of the nearby existing wells during the drilling process. There are two relatively mature techniques in case ranging. One is to put a transmission line (or a transmitter) into the wells to be located, and place a receiver on the pipe of the drilling well. The transmission line (or transmitter) generates an EM field (or static magnetic field) in the formation, which is measured by the receiver, and is used to determine the location of the already existing well. This method has a long detection distance, but it requires complicated operations and is expensive. The other technique is to place the transmitter and the receiver both on the pipe of the drilling well. The transmitter excites an EM field in the formation, which will generates an induced current in the casing of the already existing wells. The induced current produces a secondary EM field in the formation, which is detected by the receiving device. The secondary EM filed will be used to determine the locations of the already existing wells. This method is relatively simple, but its detection range is greatly reduced.